There is an increasing need for light-weight, high-output power supplies for powering the increasing supply of portable electronic equipment such as personal computers. Electrochemical cells are commonly used as power supplies for a variety of applications but are often impractical for use with portable electronic equipment because the electrical energy densities of the electrochemical cells are too low. In other words, conventional electrochemical cells that produce the desired power output are often too heavy for use with portable equipment.
One electrochemical cell that is an exception is the metal-air cell. Metal-air cells have a relatively high energy density because the cathode of a metal-air cell utilizes oxygen from ambient air as a reactant in the electrochemical reaction rather than a heavier material such as a metal or metallic composition. This results in a relatively light-weight power supply.
Metal-air cells include an air permeable cathode and a metallic anode surrounded by an aqueous electrolyte. For example, in a zinc-air cell, the anode contains zinc, and during operation, oxygen from the ambient air is converted at the cathode to hydroxide, zinc is oxidized at the anode by the hydroxide, and water and electrons are released to provide electrical energy.
Cells that are useful for only a single discharge cycle are called primary cells, and cells that are rechargeable and useful for multiple discharge cycles are called secondary cells. Both primary and secondary metal-air cells have been developed. An electrically rechargeable metal-air cell is recharged by applying voltage between the anode and cathode of the cell and reversing the electrochemical reaction. Oxygen is discharged to the atmosphere through the air permeable cathode.
Metal-air cells are often arranged in multiple cell battery packs to provide a sufficient amount of power output. In addition, it is often necessary to expose the air cathodes of the cells to a continuous flow of air at a flow rate sufficient to achieve the desired power output. Such an arrangement is shown in U.S. Pat. No. 4,913,983 wherein a fan is used to supply a flow of air to a pack of metal-air battery cells.
One problem with metal-air cells is that the ambient humidity can cause the metal-air cell to fail. Equilibrium vapor pressure of the metal-air cell results in an equilibrium relative humidity that is typically about 45 percent. If ambient humidity is greater than the equilibrium relative humidity value for the metal-air cell, the metal-air cell will absorb water from the air through the cathode and fail due to a condition called flooding. Flooding may cause the cell to leak. If the ambient humidity is less than the equilibrium relative humidity value for the metal-air cell, the cell will release water vapor from the electrolyte through the air cathode and fail due to drying out. In most environments where a metal-air battery cell is used, failure occurs from drying out.
During operation of a metal-air cell, the electrolytic reaction produces heat and increases the temperature of the cell. The heat produced by the electrolytic reaction increases the rate of vaporization of the water contained in the cell. The air flow rate over the air cathode may be increased to cool the metal-air cell as is disclosed in U.S. Pat. No. 3,395,047; however, the increase in the air flow rate over the air cathodes increases the rate of vaporization of the water and offsets the decrease in water loss from the cooling effect.
In some conventional arrangements such as that shown in U.S. Pat. No. 4,913,983, metal-air cells are arranged inside a housing and air is circulated within the housing and around the cells. The same air circulating within the housing is used for cooling the cells and for reactant air. Although the air cools the cells to some extent, the air circulating within the housing is heated by the cells and then passes through the cells and over the cathode as reactant air. The mixing of cooling air and reactant air entering the cells increases the rate of vaporization of the water in the cells and offsets the decrease in water loss from the cooling effect.
Another problem with a metal-air battery is that contaminates in the air such as carbon dioxide, smoke, and sulfides, decrease the battery output. For example, carbon dioxide reacts with the metal hydroxide in the electrolyte. The reaction between carbon dioxide and the metal hydroxide forms a metal carbonate compound that interferes with the electrochemical reaction. The exposure of metal-air battery cells to contaminates is increased if the air flow rate over the cathodes is increased for cooling.
Drying out and flooding are even greater problems for secondary metal-air cells than for primary metal-air cells. Although ambient humidity may not be a sufficient problem to flood or dry out a cell after a single cycle, cumulative water gain or loss from a series of discharge and charge cycles can cause premature failure of a secondary metal-air cell.
Another problem with metal-air cells is electrolyte leakage which causes cell failure and corrosion of the cell surroundings. As set forth above, a metal air cell includes an air cathode, an anode, and electrolyte between the cathode and anode. In a typical metal-air cell, these components are encased between frames of a cell case. The case frames may be mechanically fastened with bolts or the like, fastened with adhesive, or welded together. In each instance, the cell case includes one or more seams through which electrolyte may eventually leak. Moreover, steps taken to ensure that the seams of a metal-air cell are liquid impermeable are often time consuming and costly.
Electrolyte also may leak through or around the air cathode. It is known to cover the air side of a cathode with a liquid impermeable but gas permeable film to control electrolyte leakage. However, leaked electrolyte can form bulges beneath such a film and block or otherwise inhibit the flow of air over the cathode, or leak around the periphery of the film.
Yet another problem encountered with metal-air cells occurs when metal-air cells are arranged within an air manager housing and especially when metal-air cells are arranged in battery packs. Metal-air cells must be arranged in an air-manager so that an appropriate stoichiometric amount of air flows relatively unobstructed over the air cathode. Preferably, air for cooling also flows adjacent the cell. The passageway for air over an air cathode is typically formed by an integral part of the cell case. When a multi-cell battery pack is formed, the cells are normally fastened together by mechanical means, adhesive or welding. The formation of metal-air cell cases, battery packs, and the associated air flow passageways with such methods is complex, time consuming and costly.
Accordingly, there is a need for a metal-air power supply with an air-manager system that provides greater control over air flow through the power supply and thereby minimizes the effect of ambient humidity and contaminates on the useable life of the power supply. In addition, there is a need for such a metal-air power supply that is not likely to leak and is relatively simple in construction and inexpensive.